Plasma processing apparatus and method, and electrode plate for plasma processing apparatus

ABSTRACT

A plasma processing apparatus includes a process container configured to have a vacuum atmosphere therein. A first upper electrode is disposed to have a ring shape and to face a target substrate placed within the process container. A second upper electrode is disposed radially inside the first upper electrode and electrically insulated therefrom. A first electric feeder is configured to supply a first RF output from a first RF power supply to the first upper electrode at a first power value. A second electric feeder branches from the first electric feeder and is configured to supply the first RF output from the first RF power supply to the second upper electrode at a second power value smaller than the first power value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of PCT Application No.PCT/JP03/15028, filed Nov. 25, 2003, which was published under PCTArticle 21(2) in Japanese.

This application is based upon and claims the benefit of priority fromprior Japanese Patent Applications No. 2002-341949, filed Nov. 26, 2002;and No. 2003-358417, filed Oct. 17, 2003, the entire contents of both ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for subjecting a targetsubstrate to a plasma process, and specifically to a plasma processingtechnique of the type that applies an RF (radio frequency) to anelectrode to generate plasma. Particularly, the present inventionrelates to a plasma processing technique utilized in a semiconductorprocess for manufacturing semiconductor devices. The term “semiconductorprocess” used herein includes various kinds of processes which areperformed to manufacture a semiconductor device or a structure havingwiring layers, electrodes, and the like to be connected to asemiconductor device, on a target substrate, such as a semiconductorwafer or a glass substrate used for an LCD (Liquid Crystal Display) orFPD (Flat Panel Display), by forming semiconductor layers, insulatinglayers, and conductive layers in predetermined patterns on the targetsubstrate.

2. Description of the Related Art

In manufacturing semiconductor devices and FPDs, plasma is often usedfor processes, such as etching, deposition, oxidation, and sputtering,so that process gases can react well at a relatively low temperature.Parallel-plate plasma processing apparatuses of the capacitive couplingtype are in the mainstream of plasma processing apparatuses of thesingle substrate type.

In general, a parallel-plate plasma processing apparatus of thecapacitive coupling type includes a process container or reactionchamber configured to reduce the pressure therein, and an upperelectrode and a lower electrode disposed therein in parallel with eachother. The lower electrode is grounded and configured to support atarget substrate (semiconductor wafer, glass substrate, or the like)thereon. The upper electrode and/or lower electrode are supplied with RFvoltage through a matching unit. At the same time, a process gas isdelivered from a showerhead provided on the upper electrode side.Electrons are accelerated by an electric field formed between the upperelectrode and lower electrode and collide with the process gas, therebyionizing the gas and generating plasma. Neutral radicals and ionsderived from the plasma are used to perform a predeterminedmicro-fabrication on the surface of the substrate. In the processdescribed above, the two electrodes function to form a capacitor.

In recent years, miniaturization proceeds in the design rules used formanufacturing processes, and thus plasma processes are required togenerate higher density plasma at a lower pressure. Under thecircumstances, there is a trend in parallel-plate plasma processingapparatuses of the capacitive coupling type described above, such thatthe RF applied to the upper electrode is selected from a range coveringhigher frequencies (for example, 50 MHz or more) than conventionalvalues (typically, 27 MHz or less). However, if the frequency of the RFapplied to the upper electrode is set higher, when the RF is suppliedfrom an RF power supply through a feed rod to the electrode backside, itis transmitted through the electrode surface by means of the skin effectand is concentrated at the central portion of the electrode bottomsurface (plasma contact surface). As a consequence, the electric fieldintensity at the central portion of the electrode bottom surface becomeshigher than the electric field intensity at the peripheral portion, sothe density of generated plasma becomes higher at the electrode centralportion than at the electrode peripheral portion. Further, since plasmaradially diffuses from a high density area to a low density area, thedistribution of the plasma changes such that the density of the plasmabecomes increasingly higher at the electrode central portion than at theelectrode peripheral portion.

In order to solve this problem, a design is known in which the bottomsurface central portion of an upper electrode is formed of a highresistivity member (for example, Jpn. Pat. Appln. KOKAI Publication No.2000-323456). According to this technique, the high resistivity memberis employed for the bottom surface central portion of an upper electrodeto consume more RF power as Joule heat there. As a consequence, theelectric field intensity on the bottom surface (plasma contact surface)of the upper electrode is more reduced at the electrode central portionthan at the electrode peripheral portion, so that the low uniformitydescribed above in plasma density is remedied.

However, the high resistivity member employed for the bottom surfacecentral portion of an upper electrode may consume too much RF power asJoule heat (energy loss). Further, the influence of inductive reactancecomponents, such as a feed rod used for a feed line to the upperelectrode, become more pronounced, if the frequency of RF power isincreased. As a consequence, a resonance point may be formed at aninconstant position on the feed line or upper electrode, and anabnormally large electric current thereby flows near the resonancepoint.

Incidentally, recent plasma processing apparatuses typically include anupper electrode provided with a number of gas through-holes, from whicha process gas is delivered toward a lower electrode, i.e., the upperelectrode is used as a so-called showerhead as well. In the case of suchan upper electrode of the showerhead type, the upper electrode isattacked and sputtered by ions from the plasma, and thus is consideredas a consumable part. Particularly, the delivery port (corner) of eachgas through-hole tends to be sputtered because of the electric fieldconcentration thereat. If the gas delivery port is scraped, gas suppliedtherefrom spreads divergently, thereby making it difficult to stablygenerate plasma. For this reason, the sputtered level (flared level) ofgas delivery ports is used as an index for the electrode service life.Accordingly, the service life of an upper electrode of the showerheadtype is shortened with increase in plasma density.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a plasma processingapparatus and method, which can improve the transmission efficiency fora plasma generation RF.

Another object of the present invention is to provide a plasmaprocessing apparatus and method, which can easily uniformize plasmadensity.

Still another object of the present invention is to provide a plasmaprocessing apparatus, a method, and an electrode plate for a plasmaprocessing apparatus, which can increase the service life of an RFelectrode supplied with a plasma generation RF.

According to a first aspect of the present invention, there is provideda plasma processing apparatus comprising:

a process container configured to have a vacuum atmosphere therein;

a first upper electrode disposed to have a ring shape and to face atarget substrate placed at a predetermined position within the processcontainer;

a second upper electrode disposed radially inside the first upperelectrode and electrically insulated therefrom;

a process gas supply system configured to supply a process gas into theprocess container;

a first RF power supply configured to output a first RF;

a first electric feeder configured to supply the first RF output fromthe first RF power supply, to the first upper electrode at a first powervalue; and

a second electric feeder branching from the first electric feeder andconfigured to supply the first RF output from the first RF power supply,to the second upper electrode at a second power value smaller than thefirst power value.

According to a second aspect of the present invention, there is provideda plasma processing method comprising:

placing a target substrate at a predetermined position within a processcontainer configured to have a vacuum atmosphere therein;

supplying a first RF output from a first RF power supply to a firstupper electrode at a first power value through a first electric feeder,the first upper electrode being disposed to have a ring shape and toface the target substrate within the process container;

while supplying the first RF to the first upper electrode, supplying thefirst RF output from the first RF power supply to a second upperelectrode at a second power value smaller than the first power valuethrough a second electric feeder branching from the first electricfeeder, the second upper electrode being disposed radially inside thefirst upper electrode and electrically insulated therefrom;

while supplying the first RF to the first and second upper electrodes,supplying a predetermined process gas into the process container to turnthe process gas into plasma near a position directly below the first andsecond upper electrodes; and

subjecting the substrate to a predetermined plasma process by theplasma.

According to a third aspect of the present invention, there is providedan electrode plate used in plasma generation, to be disposed within aprocess container of a plasma processing apparatus to face a targetsubstrate placed at a predetermined position, the electrode platecomprising;

a first upper electrode having a ring shape; and

a second upper electrode disposed radially inside the first upperelectrode and electrically insulated therefrom,

wherein an RF output from an RF power supply is supplied to the firstupper electrode through a first electric feeder, and is also supplied tothe second upper electrode through a second electric feeder branchingfrom the first electric feeder.

According to the first to third aspects, the majority of plasma isgenerated directly below the first upper electrode having a ring shapeand disposed on the outer side. The generated plasma mainly diffusestoward the inner or center side, so that the plasma density becomesuniform in the radial direction. Using the plasma thus obtained, thetarget surface of the substrate (e.g., placed on a lower electrode) issubjected to a predetermined process. By suitably adjusting the size ofthe first upper electrode relative to the substrate, the distancebetween the first upper electrode and lower electrode, or the plasmageneration rate, the plasma density can become more uniform.

The first upper electrode is preferably made of a conductor orsemiconductor having a low resistivity, so that the first RF voltageoutput from the first RF power supply suffers a smaller voltage drop orpower loss. The first upper electrode preferably has a planar area ofabout ¼ times to about 1 times that of the second upper electrode on aface opposite the substrate.

The first electric feeder is disposed to efficiently supply the first RFoutput from the first RF power supply to the first upper electrode. Thisfirst electric feeder may comprise a first cylindrical conductive membercontinuously connected to the first upper electrode in an annulardirection. In this case, a second cylindrical conductive member ispreferably disposed radially outside the first cylindrical conductivemember and connected to ground potential, so as to form a coaxial pathusing the first cylindrical conductive member as a waveguide. Thissecond cylindrical conductive member may be integrated with the processcontainer.

In order to uniformize the plasma density more reliably, the secondupper electrode is disposed radially inside the first upper electrodeand electrically insulated therefrom, at a position facing the targetsubstrate within the process container. The first RF output from thefirst RF power supply is supplied to the second upper electrode throughthe second electric feeder branching from the first electric feeder, sothat the process gas is also turned into plasma near a position directlybelow the second upper electrode. In order to set the plasma generationby this second upper electrode to be secondary, a portion of the firstelectric feeder between the branching position of the second electricfeeder and the first upper electrode preferably has an inductancesmaller than the inductance of the second electric feeder.

The second electric feeder is preferably provided with a variablecapacitor. By adjusting the capacitance of this variable capacitor, aratio (P1:P2) is flexibly controlled under a certain condition (P1>P2).This ratio (P1:P2) is the ratio between the electric field intensitynear the first upper electrode or power input amount (P1) and theelectric field intensity near the second upper electrode or power inputamount (P2). As a consequence, the ratio between the plasma generationamount (or generation rate) near the first upper electrode and theplasma generation amount (or generation rate) near the second upperelectrode can be arbitrarily controlled.

A first matching unit may be used to match the output impedance of thefirst RF power supply and the load impedance with each other.Preferably, the first RF voltage is divided into the first electricfeeder and the second electric feeder from the output terminal of thefirst matching unit to supply the first RF voltage to the first upperelectrode and the second upper electrode in parallel. In this case, areactance of the load impedance to the first RF power supply, viewingfrom the output terminal of the first matching unit, may be set to becapacitive, so as to prevent a resonance point from being formed in awaveguide constituting the load circuit.

Where a showerhead structure is adopted, a gas supply cell is preferablydisposed above the second upper electrode (on the backside or rearside), viewing from the target substrate, to receive the process gasfrom the process gas supply system. In this case, a number of gasdelivery holes may be formed in the second upper electrode to deliverthe process gas from the gas supply cell toward the lower electrode. Asdescribed above, since the first upper electrode is the main membercontributing to plasma generation, the second upper electrode is lessattacked by plasma ions. As a consequence, where the electrode is usedas a showerhead, the service life can be prolonged. Preferably, the gassupply cell is divided into a central gas cell on the inner side and aperipheral gas cell on the outer side by an annular partition member,and the process gas is supplied into the central gas cell and theperipheral gas cell at a predetermined flow-rate ratio. As aconsequence, the flow-rate ratio of delivered gas between the centralportion and peripheral portion can be arbitrarily adjusted to controlthe spatial distribution of gas molecular or radical density in theradial direction. In order to optimize the plasma process, this functionof controlling the radical density distribution may be combined with thefunction described above of controlling the plasma density distribution,which is performed by adjusting the plasma generation ratio between thefirst upper electrode and second upper electrode.

A conductive member may be disposed around the second electric feeder.Preferably, the conductive member is configured to allow an inductiveelectric current for reducing the inductance of the second electricfeeder to flow therethrough. In this case, since the inductance of thesecond electric feeder is reduced, it is possible to improve the rangeof the variable capacitor's function of adjusting the balance inelectric field intensity or input power between the first and secondupper electrodes. In order to enhance the inductance reduction effect,the conductive member is preferably annular and disposed radially insidethe first electric feeder to surround the second electric feeder.

The first upper electrode may comprise a protrusion extending downward,relative to the bottom surface of the second upper electrode. Thisprotrusion of the first upper electrode applies an electric field to theplasma generation space radially inward from around, thereby providingan effect of confining plasma to effectively improve and uniformize theplasma density. Particularly, the protruded length of the protrusion ofthe first upper electrode and the positional relationship of theprotrusion relative to the substrate in the radial direction areimportant factors that can determine the spatial distributioncharacteristic of plasma density. Preferably, the protruded length ofthe protrusion is set to be 25 mm or less. The protrusion preferably hasan inner diameter portion 24 mm to 30 mm distant from the outerperipheral edge of the substrate radially outward.

A shield member may be disposed below the first upper electrode. Wherethe first upper electrode has the protrusion, as described above, theshield member may be disposed to cover the bottom surface of theprotrusion. This shield member can shield or seal an RF discharge pathnear the first upper electrode outside the plasma generation space, soas to suppress plasma generation directly therebelow. As a consequence,it is possible to further enhance the plasma confining effect directlyabove the substrate, and to effectively prevent undesirable polymerfilms from being deposited on the process container.

The second upper electrode may be electrically connected to groundpotential through a low-pass filter which allows at least a DC to passtherethrough. This low-pass filter may include resistance to adjust a DCpotential generated in the second upper electrode. Where the resistancecomprises a variable resistor, the variable resistor is preferablyadjusted to have a resistance value in accordance with the RF powervalue applied to the first and second upper electrodes as a whole. Thesecond upper electrode may be grounded through a variable DC powersupply, so that the DC potential of the second upper electrode can bedirectly controlled by the voltage of the power supply.

According to the first to third aspects, the transmission efficiency ofthe plasma generation RF supplied to the upper electrode can be improvedin one design. Further, uniformization of the plasma density can beeasily realized in another design. Furthermore, the service life of anRF electrode supplied with a plasma generation RF can be prolonged inanother design.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a sectional side view showing a plasma etching apparatusaccording to a first embodiment of the present invention;

FIG. 2 is an enlarged partial side view showing a main part of theplasma etching apparatus shown in FIG. 1;

FIG. 3 is a circuit diagram showing an equivalent circuit of a main partof plasma generating means according to the first embodiment;

FIG. 4 is a graph showing distribution characteristics of electric fieldintensity (relative value), obtained by a function of adjusting thebalance of electric field intensity according to the first embodiment;

FIG. 5 is a graph showing ratio characteristics of electric fieldintensity, obtained by a function of adjusting the balance of electricfield intensity according to the first embodiment;

FIGS. 6A and 6B are graphs showing spatial distribution characteristicsof electron density according to the first embodiment;

FIGS. 7A and 7B are graphs showing spatial distribution characteristicsof etching rate according to the first embodiment;

FIG. 8 is a sectional side view showing a plasma etching apparatusaccording to a second embodiment of the present invention;

FIGS. 9A and 9B are graphs showing spatial distribution characteristicsof etching rate according to the second embodiment;

FIGS. 10A and 10B are graphs showing spatial distributioncharacteristics of etching rate according to the second embodiment;

FIG. 11 is a graph showing characteristics of variable capacitance vs.inner input power according to the second embodiment;

FIG. 12 is a circuit diagram showing an equivalent circuit of a plasmageneration RF feed circuit according to the second embodiment;

FIG. 13 is a view showing an effect of a conductive member disposedaround an upper feed rod according to the second embodiment;

FIG. 14 is a graph showing characteristics of variable capacitance vs.bottom self-bias voltage according to the second embodiment;

FIGS. 15A and 15B are circuit diagrams each showing the circuitstructure of a low-pass filter according to the second embodiment;

FIG. 16 is a diagram showing an effect of resistance provided in alow-pass filter according to the second embodiment;

FIG. 17 is a graph showing the optimum range of resistance provided in alow-pass filter according to the second embodiment;

FIG. 18 is a sectional side view showing a main part of the plasmaetching apparatus according to the second embodiment;

FIGS. 19A to 19E are graphs showing spatial distribution characteristicsof electron density, using as parameters the inner diameter andprotruded length of an upper electrode protrusion according to thesecond embodiment;

FIGS. 20A and 20B are graphs showing characteristic lines of electrondensity uniformity, using as two-dimensional parameters the innerdiameter and protruded length of an upper electrode protrusion accordingto the second embodiment;

FIG. 21 is a sectional side view showing a plasma etching apparatusaccording to a third embodiment of the present invention;

FIGS. 22A and 22B are graphs showing spatial distributioncharacteristics of electron density to demonstrate an effect of a shieldmember according to the third embodiment;

FIG. 23 is a graph showing spatial distribution characteristics ofelectron density, using inner/outer input power ratio as a parameter,according to a fourth embodiment of the present invention;

FIG. 24 is a graph showing spatial distribution characteristics ofpolymer film deposition rate, using inner/outer input power ratio as aparameter, according to the fourth embodiment;

FIG. 25 is a graph showing spatial distribution characteristics ofetching depth, using inner/outer input power ratio as a parameter,according to the fourth embodiment;

FIG. 26 is a graph showing spatial distribution characteristics of CF₂radical density, using center/periphery gas flow-rate ratio as aparameter, according to a fifth embodiment of the present invention;

FIG. 27 is a graph showing spatial distribution characteristics of Arradical density, using center/periphery gas flow-rate ratio as aparameter, according to the fifth embodiment;

FIG. 28 is a graph showing spatial distribution characteristics of N₂radical density, using center/periphery gas flow-rate ratio as aparameter, according to the fifth embodiment;

FIG. 29 is a graph showing spatial distribution characteristics of SiF₄reaction products, using center/periphery gas flow-rate ratio as aparameter, according to the fifth embodiment;

FIG. 30 is a graph showing spatial distribution characteristics of COreaction products, using center/periphery gas flow-rate ratio as aparameter, according to the fifth embodiment;

FIG. 31 is a diagram showing a mechanism of radical generation(dissociation) obtained by a simulation according to the fifthembodiment;

FIGS. 32A to 32C are views showing an examination model for BARC etchingand sets of measurement data according to a sixth embodiment of thepresent invention;

FIGS. 33A to 33C are views showing an examination model for SiO₂ etchingand sets of measurement data according to a seventh embodiment of thepresent invention; and

FIG. 34 is a diagram showing an application example of independentcontrol over two systems for plasma density distribution and radicaldensity distribution.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described hereinafter withreference to the accompanying drawings. In the following description,the constituent elements having substantially the same function andarrangement are denoted by the same reference numerals, and a repetitivedescription will be made only when necessary.

FIRST EMBODIMENT

FIG. 1 is a sectional side view showing a plasma etching apparatusaccording to a first embodiment of the present invention. This plasmaetching apparatus is structured as a parallel-plate plasma etchingapparatus of the capacitive coupling type. The apparatus includes acylindrical chamber (process container) 10, which is made of, e.g.,aluminum with an alumite-processed (anodized) surface. The chamber 10 isprotectively grounded.

A columnar susceptor pedestal 14 is disposed on the bottom of thechamber 10 through an insulating plate 12 made of, e.g., a ceramic. Asusceptor 16 made of, e.g., aluminum is disposed on the susceptorpedestal 14. The susceptor 16 is used as a lower electrode, on which atarget substrate, such as a semiconductor wafer W, is placed.

The susceptor 16 is provided with an electrostatic chuck 18 on the top,for holding the semiconductor wafer W by an electrostatic attractionforce. The electrostatic chuck 18 comprises an electrode 20 made of aconductive film, and a pair of insulating layers or insulating sheetssandwiching the electrode 20. The electrode 20 is electrically connectedto a direct-current (DC) power supply 22. With a DC voltage applied fromthe DC power supply 22, the semiconductor wafer W is attracted and heldon the electrostatic chuck 18 by the Coulomb force. A focus ring madeof, e.g., silicon is disposed on the top of the susceptor 16 to surroundthe electrostatic chuck 18 to improve etching uniformity. A cylindricalinner wall member 26 made of, e.g., quartz is attached to the side ofthe susceptor 16 and susceptor pedestal 14.

The susceptor pedestal 14 is provided with a cooling medium space 28formed therein and annularly extending therethrough. A cooling mediumset at a predetermined temperature, such as cooling water, is circulatedwithin the cooling medium space 28 from a chiller unit (not shown)through lines 30 a and 30 b. The temperature of the cooling medium isset to control the process temperature of the semiconductor wafer Wplaced on the susceptor 16. Further, a heat transmission gas, such as Hegas, is supplied from a heat transmission gas supply unit (not shown),through a gas supply line 32, into the interstice between the topsurface of the electrostatic chuck 18 and the bottom surface of thesemiconductor wafer W.

An upper electrode 34 is disposed above the susceptor 16 in parallelwith the susceptor. The space between the electrodes 16 and 34 is usedas a plasma generation space. The upper electrode 34 defines a surfacefacing the semiconductor wafer W placed on the susceptor (lowerelectrode) 16, and thus this facing surface is in contact with plasmageneration space. The upper electrode 34 comprises an outer upperelectrode 36 and an inner upper electrode 38. The outer upper electrode36 has a ring shape or doughnut shape and is disposed to face thesusceptor 16 at a predetermined distance. The inner upper electrode 38has a circular plate shape and is disposed radially inside the outerupper electrode 36 while being insulated therefrom. In terms of plasmageneration, the outer upper electrode 36 mainly works for it, and theinner upper electrode 38 assists it.

FIG. 2 is an enlarged partial side view showing a main part of theplasma etching apparatus shown in FIG. 1. As shown in FIG. 2, the outerupper electrode 36 is separated from the inner upper electrode 38 by anannular gap (slit) of, e.g., 0.25 to 2.0 mm, in which a dielectric body40 made of, e.g., quartz is disposed. A ceramic body may be disposed inthis gap. The two electrodes 36 and 38 with the dielectric body 40sandwiched therebetween form a capacitor. The capacitance C₄₀ of thiscapacitor is set or adjusted to be a predetermined value, on the basisof the size of the gap and the dielectric constant of the dielectricbody 40. An insulating shield member 42 made of, e.g., alumina (Al₂O₃)and having a ring shape is airtightly interposed between the outer upperelectrode 36 and the sidewall of the chamber 10.

The outer upper electrode 36 is preferably made of a conductor orsemiconductor, such as silicon, having a low resistivity to generate asmall Joule heat. The outer upper electrode 36 is electrically connectedto a first RF power supply 52 through a matching unit 44, an upper feedrod 46, a connector 48, and a feed cylinder 50. The first RF powersupply 52 outputs an RF voltage with a frequency of 13.5 MHz or more,such as 60 MHz. The matching unit 44 is arranged to match the loadimpedance with the internal (or output) impedance of the RF power supply52. When plasma is generated within the chamber 10, the matching unit 44performs control for the load impedance and the output impedance of theRF power supply 52 to apparently agree with each other. The outputterminal of the matching unit 44 is connected to the top of the upperfeed rod 46.

The feed cylinder 50 has a cylindrical or conical shape, or a shapeclose to it, and formed of a conductive plate, such as an aluminum plateor copper plate. The bottom end of the feed cylinder 50 is connected tothe outer upper electrode 36 continuously in an annular direction. Thetop of the feed cylinder 50 is electrically connected to the bottom ofthe upper feed rod 46 through the connector 48. Outside the feedcylinder 50, the sidewall of the chamber 10 extends upward above theheight level of the upper electrode 34 and forms a cylindrical groundedconductive body 10 a. The top of the cylindrical grounded conductivebody 10 a is electrically insulated from the upper feed rod 46 by atube-like insulating member 54. According to this design, the loadcircuit extending from the connector 48 comprises a coaxial path formedof the feed cylinder 50 and outer upper electrode 36 and the cylindricalgrounded conductive body 10 a, wherein the former members (36 and 50)function as a waveguide.

Returning to FIG. 1, the inner upper electrode 38 includes an electrodeplate 56 having a number of gas through-holes 56 a, and an electrodesupport 58 detachably supports the electrode plate 56. The electrodeplate 56 is made of a semiconductor material, such as Si or SiC, whilethe electrode support 58 is made of a conductor material, such asaluminum with an alumite-processed surface. The electrode support 58 hastwo gas supply cells, i.e., a central gas supply cell 62 and aperipheral gas supply cell 64, formed therein and separated by anannular partition member 60, such as an O-ring. The central gas supplycell 62 is connected to some part of a number of gas delivery holes 56 aformed at the bottom surface, so as to constitute a central showerhead.The peripheral gas supply cell 64 is connected to other part of a numberof gas delivery holes 56 a formed at the bottom surface, so as toconstitute a peripheral showerhead.

The gas supply cells 62 and 64 are supplied with a process gas from acommon process gas supply source 66 at a predetermined flow-rate ratio.More specifically, a gas supply line 68 is extended from the process gassupply source 66 and divided into two lines 68 a and 68 b connected tothe gas supply cells 62 and 64. The branch lines 68 a and 68 b areprovided with flow control valves 70 a and 70 b disposed thereon,respectively. The conductance values of the flow passages from theprocess gas supply source 66 to the gas supply cells 62 and 64 are equalto each other. Accordingly, the flow-rate ratio of the process gassupplied into the two gas supply cells 62 and 64 is arbitrarily adjustedby adjusting the flow control valves 70 a and 70 b. The gas supply line68 is provided with a mass-flow controller (MFC) 72 and a switchingvalve 74 disposed thereon.

The flow-rate ratio of the process gas supplied into the central gassupply cell 62 and peripheral gas supply cell 64 is thus adjusted. As aconsequence, the ratio (FC/FE) between the gas flow rate FC from thecentral showerhead and the gas flow rate FE from the peripheralshowerhead is arbitrarily adjusted. As described above, the centralshowerhead is defined by gas through-holes 56 a at the electrode centralportion corresponding to the central gas supply cell 62, while theperipheral showerhead is defined by gas through-holes 56 a at theelectrode peripheral portion corresponding to the peripheral gas supplycell 64. Further, flow rates per unit area may be set different, for theprocess gas delivered from the central showerhead and peripheralshowerhead. Furthermore, gas types or gas mixture ratios areindependently or respectively selected, for the process gas deliveredfrom the central showerhead and peripheral showerhead.

The electrode support 58 of the inner upper electrode 38 is electricallyconnected to the first RF power supply 52 through the matching unit 44,upper feed rod 46, connector 48, and lower feed cylinder 76. The lowerfeed cylinder 76 is provided with a variable capacitor 78 disposedthereon, for variable adjusting capacitance.

Although not shown, the outer upper electrode 36 and inner upperelectrode 38 may be provided with a suitable cooling medium space orcooling jacket (not shown) formed therein. A cooling medium is suppliedinto this cooling medium space or cooling jacket from an externalchiller unit to control the electrode temperature.

An exhaust port 80 is formed at the bottom of the chamber 10, and isconnected to an exhaust unit 84 through an exhaust line 82. The exhaustunit 84 includes a vacuum pump, such as a turbo molecular pump, toreduce the pressure of the plasma process space within the chamber 10 toa predetermined vacuum level. A transfer port for a semiconductor waferW is formed in the sidewall of the chamber 10, and is opened/closed by agate valve 86 attached thereon.

In the plasma etching apparatus according to this embodiment, thesusceptor 16 used as a lower electrode is electrically connected to asecond RF power supply 90 through a matching unit 88. The second RFpower supply 90 outputs an RF voltage with a frequency of from 2 to 27MHz, such as 2 MHz. The matching unit 88 is arranged to match the loadimpedance with the internal (or output) impedance of the RF power supply90. When plasma is generated within the chamber 10, the matching unit 88performs control for the load impedance and the internal impedance ofthe RF power supply 90 to apparently agree with each other.

The inner upper electrode 38 is electrically connected to a low-passfilter (LPF) 92, which prevents the RF (60 MHz) from the first RF powersupply 52 from passing through, while it allows the RF (2 MHz) from thesecond RF power supply 98 to pass through to ground. The low-pass filter(LPF) 92 is preferably formed of an LR filter or LC filter.Alternatively, only a single conductive line may be used for this,because it can give a sufficiently large reactance to the RF (60 MHz)from the first RF power supply 52. On the other hand, the susceptor 16is electrically connected to a high pass filter (HPF) 94, which allowsthe RF (60 MHz) from the first RF power supply 52 to pass through toground.

When etching is performed in the plasma etching apparatus, the gatevalve 86 is first opened, and a semiconductor wafer W to be processed istransferred into the chamber 10 and placed on the susceptor 16. Then, anetching gas (typically a mixture gas) is supplied from the process gassupply source 66 into the gas supply cells 62 and 64 at predeterminedflow rates and flow-rate ratio. At the same time, the exhaust unit 84 isused to control the pressure inside the chamber 10, i.e., the etchingpressure, to be a predetermined value (for example, within a range offrom several mTorr to 1 Torr). Further, a plasma generation RF (60 MHz)is applied from the first RF power supply 52 to the upper electrode 34(36 and 38) at a predetermined power, while an RF (2 MHz) is appliedfrom the second RF power supply 90 to the susceptor 16 at apredetermined power. Furthermore, a DC voltage is applied from the DCpower supply 22 to the electrode 20 of the electrostatic chuck 18 to fixthe semiconductor wafer W on the susceptor 16. The etching gas deliveredfrom the gas through-holes 56 a of the inner upper electrode 38 isturned into plasma by glow discharge between the upper electrode 34 (36and 38) and susceptor 16. Radicals and ions generated in this plasma areused to etch the target surface of the semiconductor wafer W.

In this plasma etching apparatus, the upper electrode 34 is suppliedwith an RF within a range covering higher frequencies (form 5 to 10 MHzor more at which ions cannot follow). As a consequence, the plasmadensity is increased with a preferable dissociation state, so that highdensity plasma is formed even under a low pressure condition.

In the upper electrode 34, the inner upper electrode 38 is also used asa showerhead directly across the semiconductor wafer W, such that theflow-rate ratio of the gas delivered from the central showerhead (62 and56 a) and peripheral showerhead (64 and 56 a) can be arbitrarilyadjusted. As a consequence, the spatial distribution of gas molecular orradical density can be controlled in the radial direction, so as toarbitrarily control the spatial distribution of an etchingcharacteristic on the basis of radicals.

Further, as described later, the upper electrode 34 is operated as an RFelectrode for plasma generation, such that the outer upper electrode 36mainly works for it, and the inner upper electrode 38 assists it. Theratio of electric field intensity applied to electrons below the RFelectrodes 36 and 38 can be adjusted by these electrodes. As aconsequence, the spatial distribution of plasma density can becontrolled in the radial direction, so as to arbitrarily and finelycontrol the spatial distribution of a reactive ion etchingcharacteristic.

It should be noted here that the control over the spatial distributionof plasma density has substantially no influence on the control over thespatial distribution of radical density. The control over the spatialdistribution of plasma density is performed by varying the ratio ofelectric field intensity or input power between the outer upperelectrode 36 and inner upper electrode 38. On the other hand, thecontrol over the spatial distribution of radical density is performed byvarying the ratio of process gas flow rate, gas density, or gas mixturebetween the central showerhead (62 and 56 a) and peripheral showerhead(64 and 56 a).

The process gas delivered from the central showerhead (62 and 56 a) andperipheral showerhead (64 and 56 a) is dissociated in an area directlybelow the inner upper electrode 38. Accordingly, even if the balance ofelectric field intensity between the inner upper electrode 38 and outerupper electrode 36 is changed, it does not have a large influence on thebalance of radical generation amount or density between the centralshowerhead (62 and 56 a) and peripheral showerhead (64 and 56 a),because both showerheads belong to the inner upper electrode 38 (withinthe same area). Thus, the spatial distribution of plasma density and thespatial distribution of radical density can be controlled substantiallyindependently of each other.

Further, the plasma etching apparatus is arranged such that most or themajority of plasma is generated directly below the outer upper electrode36, and then diffuses to the position directly below the inner upperelectrode 38. According to this arrangement, the showerhead or innerupper electrode 38 is less attacked by ions from the plasma. Thiseffectively prevents the gas delivery ports 56 a of the electrode plate56 from being progressively sputtered, thereby remarkably prolonging theservice life of the electrode plate 56, which is a replacement part. Onthe other hand, the outer upper electrode 36 has no gas delivery portsat which electric field concentration occurs. As a consequence, theouter upper electrode 36 is less attacked by ions, and thus there arisesno such a problem in that the outer upper electrode 36 shortens theservice life in place of the inner upper electrode 38.

As described above, FIG. 2 shows a main part of the plasma etchingapparatus (particularly, a main part of plasma generating means). InFIG. 2, the structure of the showerheads (56 a, 62, and 64) of the innerupper electrode 38 is not shown. FIG. 3 is a circuit diagram showing anequivalent circuit of a main part of plasma generating means accordingto the first embodiment. In this equivalent circuit, the resistance ofrespective portions is not shown.

In this embodiment, as described above, the load circuit extending fromthe connector 48 comprises a coaxial path formed of the outer upperelectrode 36 and feed cylinder 50 and the cylindrical groundedconductive body 10 a, wherein the former members (36 and 50) function asa waveguide J₀. Where the radius (outer radius) of the feed cylinder 50is a₀, and the radius of the cylindrical grounded conductive body 10 ais b, the characteristic impedance or inductance L₀ of this coaxial pathis approximated by the following formula (1).L ₀ =K×In(b/a ₀)  (1)

In this formula, K is a constant determined by the mobility anddielectric constant of a conductive path.

On the other hand, the load circuit extending from the connector 48 alsocomprises a coaxial path formed of the lower feed rod 76 and thecylindrical grounded conductive body 10 a, wherein the former member(76) functions as a waveguide J_(i). Although the inner upper electrode38 is present on the extension of the lower feed rod 76, the impedanceof lower feed rod 76 is dominant, because the difference in diametersbetween them is very large. Where the radius (outer radius) of the lowerfeed rod 76 is a_(i), the characteristic impedance or inductance L_(i)of this coaxial path is approximated by the following formula (2).L _(i) =K×In(b/a _(i))  (2)

As could be understood from the above formulas (1) and (2), the innerwaveguide J_(i) for transmitting RF to the inner upper electrode 38provides an inductance L_(i) in the same manner as a conventionalordinary RF system. On the other hand, the outer waveguide J₀ fortransmitting RF to the outer upper electrode 36 provides a very smallinductance L₀ because of a very large radius. As a consequence, in theload circuit extending from the connector 48 toward the side opposite tothe matching unit 44, RF is transmitted more easily through the outerwaveguide J₀ having a lower impedance (a smaller voltage drop). Theouter upper electrode 36 is thereby supplied with a larger RF power P₀,so the electric field intensity E₀ obtained at the bottom surface(plasma contact surface) of the outer upper electrode 36 becomes higher.On the other hand, RF is transmitted less easily through the innerwaveguide J_(i) having a higher impedance (a larger voltage drop). Theinner upper electrode 38 is thus supplied with an RF power P_(i) smallerthan the RF power P₀ supplied to the inner upper electrode 38, so theelectric field intensity E_(i) obtained at the bottom surface (plasmacontact surface) of the inner upper electrode 38 becomes lower than theelectric field intensity E₀ on the outer upper electrode 36 side.

As described above, according to this upper electrode 34, electrons areaccelerated by a stronger electric field E₀ directly below the outerupper electrode 36, while electrons are accelerated by a weaker electricfield E₀ directly below the inner upper electrode 38. In this case, mostor the majority of plasma P is generated directly below the outer upperelectrode 36, while a subsidiary part of the plasma P is generateddirectly below the inner upper electrode 38. Then, the high densityplasma generated directly below the outer upper electrode 36 diffusesradially inward and outward, so the plasma density becomes more uniformin the radial direction within the plasma process space between theupper electrode 34 and susceptor 16.

Incidentally, in the coaxial path formed of the outer upper electrode 36and feed cylinder 50 and the cylindrical grounded conductive body 10 a,the maximum transmission power Pmax depends on the radius a₀ of the feedcylinder 50 and the radius b of the cylindrical grounded conductive body10 a, and is given by the following formula (3).Pmax/E ₀ ²max=a ₀ ² [In(b/a ₀)]²/2Z ₀  (3)

In the above formula, Z₀ is the input impedance of this coaxial pathviewing from the matching unit 44, and E₀max is the maximum electricfield intensity of the RF transmission system.

In the formula (3), the maximum transmission power Pmax takes on themaximum value when (b/a₀)≈1.65. In other words, when the ratio (b/a₀) ofthe radius of the cylindrical grounded conductive body 10 a relative tothe radius of the feed cylinder 50 is about 1.65, the power transmissionefficiency of the outer waveguide J₀ is best. Accordingly, in order toimprove the power transmission efficiency of the outer waveguide J₀, theradius of the feed cylinder 50 and/or the radius of the cylindricalgrounded conductive body 10 a are selected so that the ratio (b/a₀) ispreferably set to be at least within a range of from 1.2 to 2.0, andmore preferably within a range of from 1.5 to 1.7.

According to this embodiment, the lower feed rod 76 is provided with thevariable capacitor 78 disposed thereon as means for adjusting the ratioor balance between the outer electric field intensity E₀ directly belowthe outer upper electrode 36 (or the input power P₀ into the outer upperelectrode 36 side) and the inner electric field intensity E_(i) directlybelow the inner upper electrode 38 (or the input power P_(i) into theinner upper electrode 38 side), in order to arbitrarily and finelycontrol the spatial distribution of plasma density. The capacitance C₇₈of the variable capacitor 78 is adjusted to increase or decrease theimpedance or reactance of the inner waveguide J_(i), thereby changingthe relative ratio between the voltage drop through the outer waveguideJ₀ and the voltage drop through the inner waveguide J_(i). As aconsequence, it is possible to adjust the ratio between the outerelectric field intensity E₀ (outer input power P₀) and the innerelectric field intensity E_(i) (inner input power P_(i)).

In general, the ion sheath impedance that causes an electric potentialdrop of plasma is capacitive. In the equivalent circuit shown in FIG. 3,it is assumed (constructed) that the sheath impedance capacitancedirectly below the outer upper electrode 36 is C_(po), and the sheathimpedance capacitance directly below the inner upper electrode 38 isC_(pi). Further, the capacitance C₄₀ of the capacitor formed between theouter upper electrode 36 and inner upper electrode 38 cooperates withthe capacitance C₇₈ of the variable capacitor 78 in changing the balancebetween the outer electric field intensity E₀ (outer input power P₀) andinner electric field intensity E_(i) (inner input power P_(i)). Thecapacitance C₄₀ can be set or adjusted to optimize the variablecapacitor's 78 function of adjusting the balance of electric fieldintensity (input power).

FIGS. 4 and 5 show test examples (simulation data) in relation to thevariable capacitor's 78 function of adjusting the balance of electricfield intensity according to this embodiment. FIG. 4 shows distributioncharacteristics of electric field intensity (relative value) in theradial direction of the electrode, using the capacitance C₇₈ of thevariable capacitor 78 as a parameter. FIG. 5 shows the relative ratiobetween the outer electric field intensity E₀ and inner electric fieldintensity E_(i), using the capacitance C₇₈ of the variable capacitor 78as a parameter.

In this simulation, the diameter of the semiconductor wafer W was set at200 mm, the radius of the inner upper electrode 38 (with a circularplate shape) at 100 mm, and the inner radius and outer radius of theouter upper electrode 36 (with a ring shape) at 101 mm and 141 mm,respectively. In this case, the area of the semiconductor wafer W was314 cm², the area of the inner upper electrode 38 was 314 cm² the sameas that of the wafer W, and the area of the outer upper electrode 36 was304 cm² slightly smaller than that of the wafer W. Typically, on theface opposite the wafer W, the planar area of the outer upper electrode36 is preferably set to be about ¼ times to about 1 times the planararea of the inner upper electrode 38.

As shown in FIG. 4, the outer electric field intensity E₀ directly belowthe outer upper electrode 36 is higher than the inner electric fieldintensity E_(i) directly below the inner upper electrode 38, and a largestep of electric field intensity is thereby formed near the boundarybetween the electrodes 36 and 38. Particularly, the outer electric fieldintensity E₀ directly below the outer upper electrode 36 is apt to takeon the maximum value near the boundary abutting the inner upperelectrode 38, and gradually decrease radially outward therefrom. In thisexample, as shown in FIG. 5, where the capacitance C₇₈ of the variablecapacitor 78 is changed within a range of from 180 to 350 pF, the ratioE_(i)/E₀ between the electric field intensities E_(i) and E₀ can becontinuously controlled within a range of from about 10 to 40%. WhereC₇₈ falls within a range of from 125 to 180 pF, the load circuitproduces resonance and thereby becomes uncontrollable. In principle,within a stable domain, where the capacitance C₇₈ of the variablecapacitor 78 is increased, the reactance of the inner waveguide J_(i)decreases. As a consequence, the inner electric field intensity E_(i)directly below the inner upper electrode 38 is relatively increased, sothat the ratio E_(i)/E₀ between the outer electric field intensity E₀and inner electric field intensity E_(i) is set to be higher.

According to this embodiment, the reactance of the outer waveguide J₀provided by the feed cylinder 50 can be very small, so the impedancereactance of the load circuit, viewing from the output terminal of thematching unit 44, takes on a capacitive negative value. This means thatthere is no resonance point at which reactance causes polar inversionfrom an inductive positive value to a negative value, on the waveguideextending from the output terminal of the matching unit 44 to thecapacitive ion sheath. Since no resonance point is formed, no resonanceelectric current is generated, thereby reducing the RF energy loss andensuring stable control of the plasma density distribution.

FIGS. 6A (bias in the ON-state), 6B (bias in the OFF-state), 7A (in theX direction), and 7B (in the Y direction) show examples (experimentaldata) concerning electron density distribution characteristics andetching rate distribution characteristics, obtained by the plasmaetching apparatus according to this embodiment. In this experiment, asin the electric field intensity distribution characteristics shown inFIGS. 4 and 5, the capacitance C₇₈ of the variable capacitor 78 was usedas a parameter. The electron density was measured at positions in theradial direction, using a plasma absorption probe (PAP). Further, asilicon oxide film disposed on a semiconductor wafer was etched, and theetching rate was measured at wafer positions in the radial direction.Also in this experiment, the radius of the inner upper electrode 38 wasset at 100 mm, and the inner radius and outer radius of the outer upperelectrode 36 (with a ring shape) at 101 mm and 141 mm, respectively.Principal etching conditions were as follows:

Wafer diameter=200 mm;

Pressure inside the chamber=15 mTorr;

Temperature (upper electrode/chamber sidewall/lower electrode)=60/50/20°C.;

Heat transmission gas (He gas) supply pressure (central portion/edgeportion)=15/25 Torr;

Distance between the upper and lower electrodes=50 mm;

Process gas (C₅F₈/Ar/O₂) flow rate≈20/380/20 sccm; and

RF power (60 MHz/2 MHz)≈2200 W/1500 W (C₇₈=500 pF, 1000 pF), 1800 W(C₇₈=120 pF).

Referring to FIGS. 6A and 6B, the capacitance C₇₈ of the variablecapacitor 78 was set at 120 pF so that the ratio E_(i)/E₀ between theouter electric field intensity E₀ and inner electric field intensityE_(i) was set to be higher. In this case, the distributioncharacteristic of electron density or plasma density was formed suchthat the density took on the maximum value near the electrode center,and monotonously decreased radially outward therefrom. It is thoughtthat, in this case, the plasma diffusion rate overcame the differencebetween the plasma generation rate directly below the outer upperelectrode 36 or the main plasma generation section, and the plasmageneration rate directly below the inner upper electrode 38 or subplasma generation section. As a consequence, the plasma gathered to thecentral portion from all around, and the plasma density was therebyhigher at the central portion than at the peripheral portion.

On the other hand, the capacitance C₇₈ of the variable capacitor 78 wasset at 1000 pF so that the ratio E_(i)/E₀ between the outer electricfield intensity E₀ and inner electric field intensity E_(i) was set tobe lower. In this case, the distribution characteristic of electrondensity was formed such that the density took on the maximum value onthe outer side (near a position 140 mm distant from the center) of thewafer rather than the inner side in the radial direction, and becamealmost uniform on the inner side (0 to 100 mm) of the wafer. It isthought that, this was so because the plasma generation rate directlybelow the inner upper electrode 38 increased, and plasma diffusionradially outward was thereby enhanced. In any case, the spatialdistribution characteristic of electron density or plasma density can beflexibly and finely controlled by finely adjusting the capacitance C₇₈of the variable capacitor 78 within a suitable range.

Further, the electron density at respective positions was higher to someextent in the case (FIG. 6A) with an RF bias (2 MHz) applied to thelower electrode 16, as compared to the case (FIG. 6B) without any RFbias applied to the lower electrode 16. However, their distributionpatterns were almost the same.

Referring to the experimental data shown in FIGS. 7A and 7B, differentpatterns of the spatial distribution characteristic of etching rate wereformed by adjusting the capacitance C₇₈ of the variable capacitor 78, inaccordance with the spatial distribution characteristics of electrondensity shown in FIGS. 6A and 6B. Accordingly, the spatial distributioncharacteristic of etching rate on the wafer surface can be flexibly andfinely controlled by finely adjusting the capacitance C₇₈ of thevariable capacitor 78 within a suitable range.

Further, in the plasma etching apparatus according to this embodiment,the flow-rate ratio of the gas delivered from the central portion andperipheral portion can be adjusted by the showerhead mechanism of theinner upper electrode 36, as described above. This function allows thespatial distribution characteristic of etching rate to be controlled onthe basis of radicals.

SECOND EMBODIMENT

FIG. 8 is a sectional side view showing a plasma etching apparatusaccording to a second embodiment of the present invention. In FIG. 8,the constituent elements having substantially the same arrangement andfunction as those of the apparatus according to the first embodiment(FIG. 1) are denoted by the same reference numerals.

One of the features of the second embodiment resides in that the feedcylinder 50 or transmission path for transmitting the RF from the RFpower supply 52 to the outer upper electrode 36 is made of a cast metal.This cast metal is preferably a metal having a high conductivity andworkability, such as aluminum. As one of the advantages, cast metals canrealize a low cost, and thus reduce the cost for the member to 1/7 orless of that provided by a plate material. As another advantage, castmetals can be easily integrated, and thus can reduce the number of RFconnection surfaces in the member, thereby reducing the RF loss.

Further, even where the feed cylinder 50 is made of a cast metal, the RFtransmission efficiency is not lowered. Specifically, referring to theexperimental data shown in FIGS. 9A (cast metal), 9B (plate), 10A (castmetal), and 10B (plate), there was no substantial difference in etchingrate between the case where the feed cylinder 50 was made of a platematerial, and the case where the feed cylinder 50 was made of a castmetal. FIGS. 9A and 9B show spatial distribution characteristics ofetching rate over a silicon oxide film (SiO₂). FIGS. 10A and 10B showspatial distribution characteristics of etching rate over a photo-resist(PR) film. In this test example, principal etching conditions were asfollows:

Wafer diameter=300 mm;

Pressure inside the chamber=25 mTorr;

Temperature (upper electrode/chamber sidewall/lower electrode)=60/60/20°C.;

Heat transmission gas (He gas) supply pressure (central portion/edgeportion)=15/40 Torr;

Distance between the upper and lower electrodes=45 mm;

Process gas (C₅F₈/Ar/O₂) flow rate≈30/750/50 sccm;

RF power (60 MHz/2 MHz)≈3300 W/3800 W; and

Measurement time=120 seconds.

A second feature of the second embodiment resides in that a conductivering member 100 is disposed around the feed rod 76 inside the feedcylinder 50. The main role of the conductive member 100 is to reduce theinductance around the feed rod 76 so as to improve the range of thevariable capacitor's 78 function of adjusting the balance between outerand inner input powers, as described below.

In this plasma processing apparatus, as described above, the ratiobetween the input power P₀ into the outer upper electrode 36 and theinput power P_(i) into the inner upper electrode 38 can be arbitrarilyadjusted by adjusting the capacitance C₇₈ of the variable capacitor 78.In general, the capacitance C₇₈ of the variable capacitor 78 is adjustedstepwise, using a step motor or the like. For this capacitanceadjustment, it is necessary to avoid the uncontrollable resonance domaindescribed above (in FIG. 5, 125 pF<C₇₈<180 pF). For this reason, theexperimental test examples described above according to the firstembodiment (FIGS. 6A, 6B, 7A, and 7B) mainly used a stable domain(C₇₈≧180 pF) on the right side of the resonance domain. However, thestable domain on the right side has a limit in increasing the ratio ofthe inner input power P_(i), and also entails a large power loss. On theother hand, as shown in FIGS. 4 and 5, a domain (C₇₈≦125 pF) on the leftside of the resonance domain has an advantage in increasing the ratio ofthe inner input power P_(i), as well as a smaller power loss. However,the domain on the left side of the resonance domain becomes closer tothe resonance domain, as the ratio of the inner input power P_(i) isincreased. In the case of a characteristic line with a large change rate(inclination), such as a characteristic line A shown in FIG. 11, itbecomes very difficult to perform fine adjustment immediately before theresonance domain.

In order to solve this problem, it is effective to modify thecharacteristic line of capacitance vs. inner input power ratio, asindicated with a characteristic line B shown in FIG. 11, i.e., thechange rate (inclination) in the domain on the left side of theresonance domain is set smaller so as to expand the adjusting range.Then, in order to obtain a characteristic line with a gentle and broaderinclination as indicated with the characteristic line B shown in FIG.11, it is effective to reduce the inductance L_(i) around the feed rod76, as described below.

FIG. 12 is a circuit diagram showing an equivalent circuit of a plasmageneration RF feed circuit according to the second embodiment. Thereactance ωL_(i) around the feed rod 76 always takes on an absolutevalue larger than the reactance 1/ωC₇₈ of the capacitor 78, and thus thecomposite reactance X of the inner waveguide J_(i) is always inductiveand can be expressed by X=ωL_(a). A parallel circuit formed of thisapparent inductance L_(a) and the capacitance C₄₀ falls in a resonancestate, when the susceptance 1/ωL_(a) of the inductance L_(a) and thesusceptance ωC₄₀ of the capacitance C₄₀ cancel each other to be zero,i.e., when 1/ωL_(a)=1/(ωL_(i)−1/ωC₇₈)=ωC₄₀ is satisfied. In thisformula, with decrease in L_(i), the value Of C₇₈ to establish theresonance condition described above increases, thereby approaching acharacteristic line with a gentle and broader inclination immediatelybefore the resonance domain, as indicated with the characteristic line Bshown in FIG. 11. In the equivalent circuit shown in FIG. 12, theinductance L₀ of the outer waveguide J₀ is not shown, for the sake ofsimplicity of explanation. Even if the inductance L₀ is included in thisequivalent circuit, the principle is the same.

FIG. 13 shows an effect of the conductive member 100 according to thisembodiment. When an electric current I varying with time flows throughthe feed rod 76, a loop magnetic flux B is generated around the feed rod76, and an inductive electric current i interlinked with the magneticflux B flows through the conductive member 100 due to electromagneticinduction. As a consequence, a loop magnetic flux b is generated insideand outside the conductive member 100 by the inductive electric currenti, and the magnetic flux B is cancelled by that much corresponding tothe magnetic flux b inside the conductive member 100. Thus, theconductive member 100 disposed around the feed rod 76 can reduce the netmagnetic flux generation amount around the feed rod 76, thereby reducingthe inductance L_(i).

As regards the appearance structure of the conductive member 100, asingle ring shape continuous in an annular direction is preferably used,but a plurality of conductive members disposed in an annular directionmay be used instead. As regards the inner structure of the conductivemember 100, a hollow ring body with a cavity shown in FIG. 13 may beused, but a solid block structure, as shown in FIG. 8, can provide alarge inductance reduction effect. The volume of the conductive member100 is preferably set larger, and is ideally or most preferably set tofill the space inside the feed cylinder 50. In practice, however, it ispreferable for the conductive member 100 to occupy 1/10 to ⅓ of thespace surrounded by the feed cylinder 50 and outer upper electrode 36.The conductive member 100 is made of an arbitrary conductor material,such as aluminum or a cast metal. The conductive member 100 is disposedwhile being electrically insulated from conductive bodies around it,such as the feed rod 76 and inner upper electrode 38.

FIG. 14 shows experimental data of demonstration examples, in relationto the broadening effect described above of the conductive member 100according to this embodiment. Referring to FIG. 14, a characteristicline B′ was obtained by the apparatus structure according to thisembodiment, and a characteristic line A′ was obtained by the apparatusstructure with no conductive member 100. These characteristic lines A′and B′ have upside-down shapes of the characteristic lines A and B shownin FIG. 11, respectively. Specifically, in a parallel-plate plasmaapparatus of this type, as the ratio of the input power (inner inputpower P_(i)) into the central portion of the upper electrode 34 isincreased, the plasma density becomes higher near the substrate W on thesusceptor 16, and the bias frequency Vpp on the susceptor 16 (in inverseproportion to the plasma density) decreases. According to thisrelationship, measurement values of Vpp obtained by respective stepvalues of the variable capacitor 78 (a control variable in proportion tothe value of the capacitance C₇₈) were plotted to obtain thecharacteristic lines A′ and B′, which thus have upside-down shapes ofthe characteristic lines A and B shown in FIG. 11, respectively.According to this embodiment, thanks to the conductive member 100disposed around the feed rod 76, when the balance between the outer andinner input powers is adjusted by the variable capacitor 78, the ratioof inner input power P_(i) can be stably and finely controlled to avalue as high as possible immediately before the resonance domain, asdemonstrated by the characteristic line B′ shown in FIG. 14.

A third feature of the second embodiment relates to a low-pass filter 92connected between the inner upper electrode 38 and ground potential. Asshown in FIG. 15A, the low-pass filter 92 according to this embodimentis formed of a variable resistor 93 and a coil 95 connected in series,and arranged not to allow the plasma generation RF (60 MHz) to passthrough, but to allow an alternating frequency of the bias RF (2 MHz) orless and DC to pass through. According to the low-pass filter 92, the DCpotential or self-bias voltage Vdc on the inner upper electrode 38 canbe adjusted by adjusting the resistance value R93 of variable resistor93.

More specifically, as shown in FIG. 16, as the resistance value R93 ofthe resistor 93 is set lower, the voltage drop through the resistor 93decreases, and the negative DC potential Vdc thereby becomes higher(closer to ground potential). Conversely, as the resistance value R93 ofthe resistor 93 is set higher, the voltage drop through the resistor 93increases, and the DC potential Vdc becomes lower. If the DC potentialVdc is too high (in general, it is higher than −150V), the plasmapotential increases, thereby causing abnormal discharge or arcing. Onthe other hand, if the DC potential Vdc is too low (in general, it islower than −450V), the inner upper electrode 38 is intensely attacked byions, thereby hastening wear-out of the electrode.

In another perspective, as shown in FIG. 17, the DC potential Vdc has anappropriate range (from −450V to −150V) to prevent or suppress both ofabnormal discharge and electrode wear-out, and the resistance value R93has a range (from Ra to Rb) corresponding to this appropriate range.Accordingly, the DC potential Vdc can be adjusted to be within theappropriate range described above (from −450V to −150V) by setting oradjusting the resistance value R93 of the resistor 93 to be within therange described above (from Ra to Rb). Further, depending on the valueof an RF power applied to the entire upper electrode 34 (outer upperelectrode 36 and inner upper electrode 38), the appropriate range (fromRa to Rb) of the resistance value R93 changes. For example, in oneexperiment, the lower limit resistance value Ra=about 1 Mω was providedby an RF power of 3000 W.

Further, as shown in FIG. 15B, the inner upper electrode 38 may beconnected to ground through a variable DC power supply 97 to directlycontrol the DC potential Vdc by the voltage of the power supply. Thevariable DC power supply 97 is preferably formed of a bipolar powersupply.

A fourth feature of the second embodiment resides in that, in the upperelectrode 34, the bottom surface of the outer upper electrode 36 isprotruded downward, i.e., toward the susceptor 16, more than the bottomsurface of the inner upper electrode 38. FIG. 18 is a sectional sideview showing a main part of the plasma etching apparatus according tothe second embodiment. In this example, the outer upper electrode 36 isformed of two parts divided in the vertical direction, i.e., an upperfirst electrode member 36A and a lower second electrode member 36B. Themain body or first electrode member 36A is made of, e.g.,alumite-processed aluminum, and is connected to the feed cylinder 50.The replacement part or second electrode member 36B is made of, e.g.,silicon, and is detachably fixed to and in close contact with the firstelectrode member 36A by bolts (not shown). The second electrode member36B is protruded by a predetermined value H, relative to the bottomsurface of the inner upper electrode 38. A member 102 for enhancingthermal conductance, such as a silicone rubber sheet, is interposedbetween two electrode members 36A and 36B. The contact surfaces of thetwo electrode members 36A and 36B may be coated with Teflon™ to reducethermal resistance.

In the outer upper electrode 36, the protruded length H and innerdiameter Φ of the protrusion part 36B can define the intensity ordirection of an electric field provided from the outer upper electrode36 or upper electrode 34 to the plasma generation space. Thus, they areimportant factors to thereby determine the spatial distributioncharacteristic of plasma density.

FIGS. 19A to 19E show examples (experimental data) concerning spatialdistribution characteristics of electron density, using as parametersthe protruded length H and inner diameter Φ of the protrusion part 36B.Also in this experiment, the electron density was measured at positionsin the radial direction, using a plasma absorption probe (PAP). Thediameter of a semiconductor wafer was set at 300 mm. As regards the mainparameters Φ and H, the experimental examples shown in FIG. 19A employedΦ=329 mm and H=15 mm, the experimental examples shown in FIG. 19Bemployed Φ=329 mm and H=20 mm, the experimental examples shown in FIG.19C employed Φ=339 mm and H=0.20 mm, the experimental examples shown inFIG. 19D employed Φ=349 mm and H=20 mm, and the experimental examplesshown in FIG. 19E employed Φ=359 mm and H=25 mm. As a secondaryparameter, the ratio P_(i)/P₀ (RF power ratio) between the inner inputpower P_(i) and outer input power P₀ was set at four different values,i.e., (30/70), (27/73), (20/80), and (14/86).

In the experimental data shown in FIGS. 19A to 19E, an inflection pointF at which the electron density quickly drops was moved radially outwardwith increase in the inner diameter Φ of the protrusion part 36B of theouter upper electrode 36, and was moved up with increase in theprotruded length H of the protrusion part 36B. The ideal distributioncharacteristic is a characteristic in which an inflection point F islocated directly above the wafer edge (a position of 150 mm), and thedensity is flat at a high value between the center and edge. In light ofthis, one characteristic (Φ=349 mm and H=20 mm) shown in FIG. 19Dobtained by an RF power ratio P_(i)/P₀ of 30/70 is closest to the idealstate.

FIG. 20A shows characteristics of total uniformity U_(T) and edgeuniformity U_(E) in the spatial distribution of electron density, usingΦ and H as two-dimensional parameters. The total uniformity U_(T) standsfor planar uniformity over the entire region in the radial directionfrom the wafer central position (R₀) to the wafer edge position (R₁₅₀),as shown in FIG. 20B. The edge uniformity U_(E) stands for planaruniformity over a region near the wafer edge, such as a region from aposition of radius 130 mm (R₁₃₀) to the wafer edge position (R₁₅₀).

As understood from the characteristics shown in FIG. 20A, the protrudedlength H of the protrusion part 36B is remarkably influential on thetotal uniformity U_(T), and also has a large effect on the edgeuniformity U_(E). On the other hand, the inner diameter Φ of theprotrusion part 36B is influential on the edge uniformity U_(E), but isscarcely influential on the total uniformity U_(T). In total, theprotruded length H of the protrusion part 36B is preferably set at 25 mmor less, and most preferably set to be near 20 mm. The inner diameter Φof the protrusion part 36B is preferably set to be within a range offrom 348 mm to 360 mm, and most preferably set to be near 349 mm. Therange of from 348 mm to 360 mm means that the protrusion part 36B isdisposed at a position 24 mm to 30 mm distant from the wafer edgeradially outward.

It should be noted that the protrusion part 36B of the outer upperelectrode 36 applies an electric field to the plasma generation spaceradially inward from around, thereby providing an effect of confiningplasma. For this reason, the protrusion part 36B is disposed preferablyor almost essentially at a position outside the wafer edge in the radialdirection, in order to improve uniformity in the spatial distributioncharacteristic of plasma density. On the other hand, the width of theprotrusion part 36B in the radial direction is not important, and thuscan be arbitrarily set.

THIRD EMBODIMENT

FIG. 21 is a sectional side view showing a main part of a plasma etchingapparatus according to a third embodiment of the present invention. Theparts of this apparatus can be the same as those of the secondembodiment except for the featuring parts. The third embodiment has afeature in that a shield member 104 is disposed along the protrusionpart 36B of the outer upper electrode 36 according to the secondembodiment.

For example, the shield member 104 is formed of an aluminum plate withan alumite-processed surface, and physically and electrically coupled tothe sidewall of the process container 10. The shield member 104 extendsessentially in the horizontal direction from the container sidewall tothe position below the protrusion part 36B of the outer upper electrode36 to cover the bottom surfaces of the protrusion part 36B and thering-shaped shield member 42 in a non-contacting or insulated state. Thesecond electrode member 36B of the outer upper electrode 36 has anL-shaped cross section with a peripheral portion extending downward inthe vertical direction to form a protrusion. The protruded length H andinner diameter Φ of this protrusion can be defined in accordance withthe same numerical conditions as those of the second embodiment.

A function of the shield member 104 is to shield or seal RF dischargefrom the bottom surfaces of the protrusion part 36B of the outer upperelectrode 36 and ring-shaped shield member 42, so as to suppress plasmageneration directly below them. As a consequence, it is possible toprimarily enhance the plasma confining effect directly above the wafer.

FIGS. 22A (with the shield member) and 22B (without the shield member)show experimental data concerning the plasma confining effect providedby the shield member 104. Where the shield member 104 was not used, asshown in FIG. 22B, the plasma electron density once dropped and thenincreased again to form a peak outside the wafer edge position (150 mm)in the radial direction. This was so because RF power was dischargedvertically downward from the bottom surfaces of the protrusion part 36Bof the outer upper electrode 36 and ring-shaped shield member 42,whereby plasma was also generated directly below them and thus electronsand ions were present there. Since a certain amount of plasma waspresent in a space distant from the wafer edge position radiallyoutward, the plasma density directly above the wafer decreased by thatmuch.

On the other hand, where the shield member 104 was used according tothis embodiment, as shown in FIG. 22A, the electron density (plasmadensity) essentially monotonously decreased radially outward outside thewafer edge position (150 mm), while it was higher directly above thewafer as a whole. This was so because the bottom surfaces of theprotrusion part 36B of the outer upper electrode 36 and ring-shapedshield member 42 did not work as an RF path any more due to the presenceof the shield member 104, whereby plasma generation directly below themwas remarkably reduced. The plasma confining effect or plasmadiffusion-preventing effect of the shield member 104 was enhanced, withincrease in the RF power of the RF power supply 52.

Further, as a secondary effect, where plasma generation is remarkablyreduced by the shield member 104 outside the wafer edge position, asdescribed above, etching species, such as radicals and ions, arereduced. As a consequence, it is possible to effectively preventundesirable polymer films from being deposited on portions inside thecontainer (particularly near the shield member 104).

For example, conventionally, where a Low-k film (an inter-levelinsulating film with a low dielectric constant) is etched, a plasmaetching is performed, and then ashing (resist removal) is performedusing O₂ gas within the same chamber. At this time, reactive species(such as CE and F), which have been deposited as polymers inside thecontainer during the previous plasma etching, are activated by activeoxygen atoms in plasma, and cause damage to the Low-k film (Low-kdamages), such that they etch the via-holes of the film into a bowingshape or invade the film and change its k value. According to thisembodiment, however, the shield member 104 can effectively preventundesirable deposition of reactive species during plasma etching,thereby solving problems concerning Low-k damages described above. Theshield member 104 may be made of an arbitrary conductor or semiconductor(such as, silicon), or a combination of different materials.

FIG. 21 also shows an arrangement of cooling medium passages 106 and 108formed in the upper electrode 34 (36 and 38). A cooling medium set at anadjusted temperature is circulated within each of the cooling mediumpassages 106 and 108 from a chiller unit (not shown) through lines 110and 112. Specifically, the cooling medium passages 106 are formed in thefirst electrode member 36A of the outer upper electrode 36. Since thesecond electrode member 36B is coupled with the first electrode member36A through a coating or sheet 102 for enhancing thermal conductance, itcan also be effectively cooled by the cooling mechanism.

The electrodes are supplied with a cooling medium even when the RF powersupplies 52 and 90 are in the OFF-state. Conventionally, the plasmaprocessing apparatus of this type employs an insulative cooling medium,such as Galden™. In this case, when the cooling medium flows through acooling medium passage, it generates an electrostatic charge byfriction, by which the electrode enters an abnormally high voltagestate. If an operator's hand touches the electrode in this state duringa maintenance operation or the like in which the RF power supplies arein the OFF-state, the operator may get an electric shock. However, theplasma processing apparatus according to this embodiment allowselectrostatic charge generated in the inner upper electrode 38 to bereleased to ground through the resistor 93 of the low-pass filter 92(see FIG. 8), whereby an operator is prevented from getting an electricshock.

FOURTH EMBODIMENT

Using the plasma etching apparatus according to the third embodiment(FIGS. 8 and 21), an experiment was conducted of etching a silicon oxidefilm (SiO₂) to form a hole having a diameter (Φ) of 0.22 μm. In thisexperiment, etching characteristics (particularly etching rate) wereexamined, using as a parameter the RF power input ratio (P_(i)/P₀)between the outer upper electrode 36 and inner upper electrode 38. Otheretching conditions are shown below. FIGS. 23 to 25 show experimentalresult data.

Wafer diameter=300 mm;

Pressure inside the chamber=20 mTorr;

Temperature (upper electrode/chamber sidewall/lower electrode)=20/60/60°C.;

Heat transmission gas (He gas) supply pressure (central portion/edgeportion)=20/35 Torr;

Distance between the upper and lower electrodes=45 mm;

Protruded length (H) of the outer upper electrode=15 mm;

Process gas (C₅F₈/CH₂F₂/N₂/Ar/O₂)≈10/20/110/560/10 sccm;

RF power (60 MHz/2 MHz)≈2300 W/3500 W; and

Etching time=120 seconds.

As shown in FIG. 23, as the ratio of the inner input power P_(i)increased, i.e., 14%, 18%, and 30%, the electron density or plasmadensity became higher in proportion to the P_(i) ratio near the wafercentral portion, while it did not change near wafer edge portion.Accordingly, where the RF power input ratio (P_(i)/P₀) is adjusted onthe basis of this, the spatial distribution characteristic of plasmadensity can be controlled in the radial direction.

FIG. 24 shows the measurement result of deposition rate of a polymerfilm, formed from reaction products and reactive species, at respectivepositions in the radial direction, wherein the deposition rate is inproportion to radical density. This experiment was conducted to see theeffect of change in the RF power input ratio (P_(i)/P₀) on the radicaldensity. A bare silicon wafer was used as a sample substrate on whichthe polymer film was deposited. As indicated by the experimental datashown in FIG. 24, change in the RF power input ratio (P_(i)/P₀) only hada very small influence on the polymer film deposition rate i.e., thespatial distribution characteristic of radical density.

FIG. 25 shows etching depth measured at respective positions of thewafer in the radial direction, obtained by the SiO₂ etching describedabove. As shown in FIG. 25, as the ratio of the inner input power P_(i)increased, i.e., 14%, 18%, and 30%, the etching depth became larger inproportion to the P_(i) ratio near the wafer central portion, while itdid not differ so much near wafer edge portion. This was similar to thatof the electron density (FIG. 24).

As described above, judging from the experimental data shown in FIGS. 23to 25, the following matters have been confirmed. Specifically, byadjusting the RF power input ratio (P_(i)/P₀) between the outer upperelectrode 36 and inner upper electrode 38, the spatial distributioncharacteristic of plasma density in the radial direction can becontrolled without substantially affecting the spatial distributioncharacteristic of radical density, i.e., independently of the controlover the spatial distribution of radical density. Accordingly, theuniformity of etching depth i.e., etching rate, can be improved byadjusting the RF power input ratio (P_(i)/P₀). It should be noted that,if the plasma etching apparatus according to the first or secondembodiment (FIGS. 1, 8, and 18) is used, the same experimental result asdescribed above is obtained.

FIFTH EMBODIMENT

Using the plasma etching apparatus according to the third embodiment(FIGS. 8 and 21), a simulation was conducted of etching a silicon oxidefilm (SiO₂) with a CF family process gas. In this experiment, thedistributions of radicals or reaction products were examined, using as aparameter the ratio (FC/FE) between the flow rate FC of a process gasdelivered from the central showerhead (62 and 56 a) and the flow rate FEof the process gas delivered from the peripheral showerhead (64 and 56a). In this simulation, it was assumed that neither reaction norabsorption of reaction products or reactive species was caused on thewafer surface, but the following reaction was simply caused on theblanket SiO₂ film.2CF₂+SiO₂→SiF₄+2CO

Other principal etching conditions are shown below. FIGS. 26 to 30 showsimulation result concerning radicals and reaction products. FIG. 31shows the type and generation rate (denoted by a percentage value inbrackets) of radicals generated by gradual dissociation from moleculesof the main etching gas (C₄F₈).

Wafer diameter=200 mm;

Pressure inside the chamber=50 mTorr;

Temperature (upper electrode/chamber sidewall/lower electrode)=20/60/60°C.;

-   -   Heat transmission gas (He gas) supply pressure (central        portion/edge portion)=10/35 Torr;

Distance between the upper and lower electrodes=30 mm;

-   -   Protruded length (H) of the outer upper electrode=15 mm;

Process gas (C₄F₈/N₂/Ar)≈5/120/1000 sccm; and

RF power (60 MHz/2 MHz)≈1200 W/1700 W.

As shown in FIG. 26, the distribution characteristic of CF₂ density,which is the main reactive species, was remarkably influenced by the gasflow-rate ratio (FC/FE) between the center and periphery. Specifically,as the ratio of the central gas flow rate FC was increased, the CF₂density became higher near the wafer central portion while it did notchange so much near the wafer edge portion. As shown in FIG. 28, thedistribution characteristic of CO radical density also showed a similarchange with change in the gas flow-rate ratio (FC/FE) between the centerand periphery. By contrast, as shown in FIG. 27, the distributioncharacteristic of Ar radical density scarcely changed with change in thegas flow-rate ratio (FC/FE) between the center and periphery.

As regards reaction products, as shown in FIGS. 29 and 30, either ofSiF₄ density and CO density was remarkably influenced by the gasflow-rate ratio (FC/FE) between the center and periphery. Morespecifically, as the ratio of the central gas flow rate FC was reduced,each of the SiF₄ density and CO density became higher near the wafercentral portion while it did not change so much near the wafer edgeportion. Where the central gas flow rate FC was equal to the peripheralgas flow rate FE (FC/FE=50/50), the density became higher near the wafercentral portion than near the wafer edge portion. As described above,reaction products tended to gather to the central side, because reactionproducts were less moved laterally by a fresh gas flow at the centralportion, as compared with the peripheral portion.

If reaction products have a non-uniform distribution on a wafer, theynot only affect the uniformity of process gas supply rate or chemicalreaction among respective positions, but also may directly affect theetching shape or selectivity. According to this embodiment, as shown inFIGS. 29 and 30, where the central gas flow rate FC is set to be higherthan the peripheral gas flow rate FE (FC/FE≈70/30 in this shownexample), the space density distribution of reaction products can becomeuniform. It should be noted that, if the plasma etching apparatusaccording to the first or second embodiment (FIGS. 1, 8, and 18) isused, the same simulation result as described above is obtained.

SIXTH EMBODIMENT

Using the plasma etching apparatus according to the third embodiment(FIGS. 8 and 21), an experiment was conducted of etching a BARC(antireflective film). In this experiment, the etching shape andselectivity were examined, using the gas flow-rate ratio (FC/FE) betweenthe center and periphery as a parameter. A test sample shown in FIG. 32Awas used. A mask having an opening diameter (Φ) of 0.12 μm, aphoto-resist film having a thickness of 350 nm, a BARC film having athickness of 80 nm, and an SiO₂ film having a thickness of 700 nm wereused. “Oxide loss” and “resist remaining amount” were measured asexamination items for the selectivity, and “bottom CD” was measured asan examination item for the etching shape or dimensional accuracy. FIG.32B shows measurement values of the respective examination items atFC/FE=50/50. FIG. 32C shows measurement values of the respectiveexamination items at FC/FE=70/30. As regards the measurement point,“center” denotes a position at the wafer center, and “edge” denotes aposition 5 mm distant from the wafer notch end toward the center.

Principal etching conditions were as follows:

Wafer diameter=300 mm;

Pressure inside the chamber=150 mTorr;

Heat transmission gas (He gas) supply pressure (central portion/edgeportion)=10/25 Torr;

Distance between the upper and lower electrodes=30 mm;

Protruded length (H) of the outer upper electrode=15 mm;

Process gas (CF4)≈200 sccm;

RF power (60 MHz/2 MHz)≈500 W/600 W; and

Etching time=30 seconds.

As regards the examination items for the BARC etching, the “oxide loss”is the etched depth of the underlying SiO₂ film provided by over etchingof the BARC etching. For this value, a smaller value is better, but apriority resides in that a smaller difference in this value over thewafer (particularly the difference between the center and edge) isbetter. The “resist remaining amount” is the thickness of thephoto-resist remaining after the etching. For this value, a larger valueis better, and a smaller difference in this value is also better. The“bottom CD” is the bottom diameter of a hole formed in the BARC. Forthis value, a value closer to the mask opening diameter Φ is better, anda smaller difference in this value is also better.

As shown in FIG. 32B, where the central gas flow rate FC was equal tothe peripheral gas flow rate FE (5:5), the differences between thecenter and edge were large in all the examination items, andparticularly the difference was large in the “resist remaining amount”.By contrast, as shown in FIG. 32B, where the central gas flow rate FCwas larger than the peripheral gas flow rate FE (7:3), all theexamination items stably took on better values and became uniformbetween the center and edge, i.e., the selectivity and etching shapewere remarkably improved.

As described above, according to this embodiment, within the processcontainer 10, particularly within the plasma generation space definedbetween the upper electrode 34 and lower electrode 16, the inner upperelectrode 38 of the upper electrode 34 is used while the ratio (FC/FE)between the process gas flow rate FC delivered from the centralshowerhead (62 and 56 a) and the process gas flow rate FE (64 and 56 a)delivered from the peripheral showerhead is adjusted. As a consequence,the spatial distribution of radical density can be controlled touniformize etching characteristics (such as the selectivity and etchingshape) on the basis of radicals. It should be noted that, if the plasmaetching apparatus according to the first or second embodiment (FIGS. 1,8, and 18) is used, the same measurement result as described above isobtained.

SEVENTH EMBODIMENT

Using the plasma etching apparatus according to the third embodiment(FIGS. 8 and 21), an experiment was conducted of etching an SiO₂ film.In this experiment, the etching shape was examined, using the gasflow-rate ratio (FC/FE) between the center and periphery as a parameter.A test sample shown in FIG. 33A was used. A mask having an openingdiameter (Φ) of 0.22 μm, a photo-resist film having a thickness of 500nm, a BARC film having a thickness of 100 nm, and an SiO₂ film having athickness of 1 μm were used. “Etching depth”, “top CD”, and “bottom CD”were measured as examination items for the etching shape. FIG. 33B showsmeasurement values of the respective examination items at FC/FE=50/50.FIG. 33C shows measurement values of the respective examination items atFC/FE=10/90. Principal etching conditions were as follows:

Wafer diameter=300 mm;

Pressure inside the chamber=20 mTorr;

Temperature (upper electrode/chamber sidewall/lower electrode)=20/60/60°C.

Heat transmission gas (He gas) supply pressure (central portion/edgeportion)=20/35 Torr;

Distance between the upper and lower electrodes=45 mm;

Protruded length (H) of the outer upper electrode=15 mm;

Process gas (C₅F₈/CH₂F₂/N₂/Ar/O₂)≈10/20/110/560/10 sccm;

RF power (60 MHz/2 MHz)≈2300 W/3500 W;

RF power ratio (inner input power P_(i)/outer input power P₀)=30:70; and

Etching time=120 seconds.

As regards the examination items for the SiO₂ etching, the “etchingdepth” is the depth of a hole formed in the SiO₂-film by the etchingtime (120 seconds), i.e., it corresponds to the etching rate. The “topCD” and “bottom CD” are the top and bottom diameters of the hole formedin the SiO₂ film, and, as these values are closer to each other, thevertical shape characteristic of the hole (anisotropy) is better. As amatter of course, for each of the examination items, it is morepreferable that the difference between the center and edge is smaller.

As shown in FIG. 33B, where the central gas flow rate FC was equal tothe peripheral gas flow rate FE (5:5), the difference was large in the“etching depth”, and values of the ratio of bottom CD/top CD atrespective positions were small, i.e., the holes were more tapered. Bycontrast, as shown in FIG. 33B, where the central gas flow rate FC wassmaller than the peripheral gas flow rate FE (1:9), the “etching depth”i.e., etching rate became more uniform, and the vertical shapecharacteristic was improved and more uniform.

As described above, also in this embodiment, by adjusting the ratio(FC/FE) between the inner gas flow rate FC and the outer gas flow rateFE, the spatial distribution of radical density can be controlled touniformize etching characteristics (particularly the etching shape) onthe basis of radicals. It should be noted that, if the plasma etchingapparatus according to the first or second embodiment (FIGS. 1, 8, and18) is used, the same measurement result as described above is obtained.

According to the embodiments described above, the plasma densitydistribution and the radical density distribution can be controlledindependently of each other within the plasma generation space definedin the process container 10. This independent control over two systemscan be used to preferably deal with various plasma process applications,such as those shown in the map of FIG. 34.

The embodiments described above may be modified in various manners, inaccordance with the technical ideas of the present invention. Forexample, only the outer upper electrode 36 may be supplied with an RFfrom the first RF power supply 52 through the matching unit 44 and feedcylinder 50, while the inner upper electrode 38 being supplied with noRF. Also in this case, the inner upper electrode 38 can function as ashowerhead or function as an electrode for an RF output from the secondRF power supply 90 to flow to ground. Alternatively, the inner upperelectrode 38 may be replaced with a single-purpose showerhead with noelectrode function. In the embodiments described above, the outer upperelectrode 36 is formed of one or single ring electrode, but it may beformed of a plurality of electrodes combined to form a ring as a whole.Further, the inner diameter of the outer upper electrode 36 may be setvery small, or the outer upper electrode 36 may be formed of a circularplate. Depending on the application, the second RF power supply 90 maybe omitted. The present invention can be applied not only to plasmaetching, but also to various plasma processes, such as plasma CVD,plasma oxidation, plasma nitridation, and sputtering. As regards atarget substrate, the present invention can be applied not only to asemiconductor wafer, but also to various substrates for a flat paneldisplay, photo mask, CD substrate, and print substrate.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventionconcept as defined by the appended claims and their equivalents.

1. A plasma processing apparatus comprising: a process containerconfigured to have a vacuum atmosphere therein; a first upper electrodedisposed to have a ring shape and to face a target substrate placed at apredetermined position within the process container; a second upperelectrode disposed radially inside the first upper electrode andelectrically insulated therefrom; a process gas supply system configuredto supply a process gas into the process container; a first RF powersupply configured to output a first RF; a first electric feederconfigured to supply the first RF output from the first RF power supply,to the first upper electrode at a first power value; and a secondelectric feeder branching from the first electric feeder and configuredto supply the first RF output from the first RF power supply, to thesecond upper electrode at a second power value smaller than the firstpower value, wherein the first upper electrode comprises a protrusionextending downward and having a protrusion surface present below abottom surface of the second upper electrode and a falling surfacepositioned radially at an inside of the protrusion and extendingdownward toward the protrusion surface such that the protrusion surfaceis disposed to be radially outside the substrate placed at thepredetermined position within the process container, wherein aninsulating member is interposed between a wall of the process containerand the first upper electrode, and wherein a shield member is disposednot to cover the falling surface but to cover the protrusion surface anda bottom surface of the insulating member so as to prevent the first RFfrom being supplied from the protrusion surface and a bottom surface ofthe insulating member into the plasma process space.
 2. The apparatusaccording to claim 1, wherein the first upper electrode is made of aconductor or semiconductor having a low resistivity.
 3. The apparatusaccording to claim 1, wherein the first upper electrode has a planararea about ¼ times to about 1 times that of the second upper electrodeon a face opposite the substrate.
 4. The apparatus according to claim 1,wherein the first upper electrode comprises a first electrode memberconnected to the first electric feeder, and a second electrode memberincluding the protrusion and detachably mounted on a bottom surface ofthe first electrode member to face the substrate.
 5. The apparatusaccording to claim 4, wherein the first electrode member has a coolingmedium passage for a cooling medium to flow therethrough.
 6. Theapparatus according to claim 4, further comprising a film or sheetdisposed between the first electrode member and the second electrodemember, and configured to reduce thermal resistance therebetween.
 7. Theapparatus according to claim 1, wherein a portion of the first electricfeeder between a branching position of the second electric feeder andthe first upper electrode has an inductance smaller than an inductanceof the second electric feeder.
 8. The apparatus according to claim 1,wherein the first electric feeder comprises a first cylindricalconductive member continuously connected to the first upper electrode inan annular direction.
 9. The apparatus according to claim 8, furthercomprising a second cylindrical conductive member disposed radiallyoutside the first cylindrical conductive member and connected to groundpotential.
 10. The apparatus according to claim 9, wherein a radiusratio of a radius of the second cylindrical conductive member relativeto a radius of the first cylindrical conductive member is set to bewithin a range of from 1.2 to 2.0.
 11. The apparatus according to claim10, wherein the radius ratio is set to be within a range of from 1.5 to1.7.
 12. The apparatus according to claim 8, further comprising aconductive member disposed to occupy 1/10 to ⅓ of a space surrounded bythe first cylindrical conductive member and the first upper electrode.13. The apparatus according to claim 1, wherein the first upperelectrode is formed of a single ring electrode.
 14. The apparatusaccording to claim 1, wherein the second electric feeder comprises avariable capacitor.
 15. The apparatus according to claim 1, furthercomprising a first matching unit connected to an output terminal of thefirst RF power supply and configured to match an output impedance of thefirst RF power supply and a load impedance with each other, wherein areactance of the load impedance to the first RF power supply, viewingfrom an output terminal of the first matching unit, is capacitive. 16.The apparatus according to claim 1, further comprising a dielectric bodydisposed in an interstice between the first upper electrode and thesecond upper electrode.
 17. The apparatus according to claim 1, whereina conductive member is disposed around the second electric feeder. 18.The apparatus according to claim 17, wherein the conductive member isconfigured to allow an inductive electric current for reducing aninductance of the second electric feeder to flow therethrough.
 19. Theapparatus according to claim 18, wherein the conductive member forms aclosed circuit around the second electric feeder, which is interlinkedwith a magnetic flux generated by an electric current flowing throughthe second electric feeder.
 20. The apparatus according to claim 19,wherein the conductive member is annularly disposed to surround thesecond electric feeder.
 21. The apparatus according to claim 20, whereinthe conductive member is formed of a single conductive ring.
 22. Theapparatus according to claim 1, wherein a protruded length of theprotrusion downward from a bottom surface of the second upper electrodeis set to be 25 mm or less.
 23. The apparatus according to claim 1,wherein the protrusion has an inner diameter portion 24 mm to 30 mmdistant from an outer peripheral edge of the substrate radially outward.24. The apparatus according to claim 1, wherein the shield member ismade of a conductor or semiconductor.
 25. The apparatus according toclaim 1, wherein the shield member extends from a position below thefirst upper electrode to an inner wall of the process container.
 26. Theapparatus according to claim 1, wherein the shield member iselectrically connected to the process container, which is electricallyseparated from the first upper electrode and is grounded.
 27. Theapparatus according to claim 1, wherein a gas supply cell is disposedabove the second upper electrode, and a number of gas delivery holes areformed in the second upper electrode to deliver the process gas from thegas supply cell toward the substrate.
 28. The apparatus according toclaim 27, wherein the gas supply cell is divided into a central gas cellon an inner side and a peripheral gas cell on an outer side by anannular partition member, and the process gas is supplied into thecentral gas cell and the peripheral gas cell at predetermined flow-rateratio.
 29. The apparatus according to claim 1, further comprising: alower electrode disposed within the process container to face the firstand second upper electrodes and configured to place the substratethereon; and a second RF power supply configured to supply the lowerelectrode with a second RF having a frequency lower than that of thefirst RF, wherein the second upper electrode is electrically connectedto ground potential through a low-pass filter which does not allow thefirst RF to pass therethrough but allows the second RF to passtherethrough.
 30. The apparatus according to claim 1, further comprisinga variable DC power supply electrically connected between the secondupper electrode and ground potential.
 31. The apparatus according toclaim 1, further comprising a low-pass filter electrically connectedbetween the second upper electrode and ground potential, and configuredto allow a DC to pass therethrough, wherein the low-pass filter includesresistance comprising a variable resistor to adjust a DC potentialgenerated in the second upper electrode.
 32. The apparatus according toclaim 31, wherein the variable resistor is adjusted to have a resistancevalue in accordance with the first and second power values.